Improve support for composite types in PL/Python.

Allow PL/Python functions to return arrays of composite types.
Also, fix the restriction that plpy.prepare/plpy.execute couldn't
handle query parameters or result columns of composite types.

In passing, adopt a saner arrangement for where to release the
tupledesc reference counts acquired via lookup_rowtype_tupdesc.
The callers of PLyObject_ToCompositeDatum were doing the lookups,
but then the releases happened somewhere down inside subroutines
of PLyObject_ToCompositeDatum, which is bizarre and bug-prone.
Instead release in the same function that acquires the refcount.

Ed Behn and Ronan Dunklau, reviewed by Abhijit Menon-Sen

src/pl/plpython/sql/plpython_spi.sql

@@ -284,6 +284,17 @@ plan = plpy.prepare("select fname, lname from users where fname like $1 || '%'",
 c = plpy.cursor(plan, ["a", "b"])
 $$ LANGUAGE plpythonu;
 
+CREATE TYPE test_composite_type AS (
+  a1 int,
+  a2 varchar
+);
+
+CREATE OR REPLACE FUNCTION plan_composite_args() RETURNS test_composite_type AS $$
+plan = plpy.prepare("select $1 as c1", ["test_composite_type"])
+res = plpy.execute(plan, [{"a1": 3, "a2": "label"}])
+return res[0]["c1"]
+$$ LANGUAGE plpythonu;
+
 SELECT simple_cursor_test();
 SELECT double_cursor_close();
 SELECT cursor_fetch();
@@ -293,3 +304,4 @@ SELECT next_after_close();
 SELECT cursor_fetch_next_empty();
 SELECT cursor_plan();
 SELECT cursor_plan_wrong_args();
+SELECT plan_composite_args();